As is known in the art, there exists a class of materials referred to as thermoelectric materials. A thermoelectric material is a type of material which can directly convert thermal energy into electrical energy or which can convert electrical energy into thermal energy.
A typical bulk thermoelectric device includes a section of P-type material that is in mechanical and electrical communication with a section of N-type material. The point at which the two materials contact each other is referred to as a “junction”. Whenever electrical current flows through two dissimilar materials, depending on the direction of current flow through the materials, the junction of the P-type and N-type material will either absorb or release heat.
When the thermoelectric device is connected to a voltage source such that the P-type material is connected to the positive lead of the voltage source and the N-type material is connected to the negative lead of the voltage source a phenomenon takes place. The negative charge carriers, also known as electrons, in the N-type material are repelled by the negative potential and attracted to the positive potential of the voltage source. Similarly, the positive charge carriers, also known as holes, in the P-type material are repelled by the positive voltage potential and attracted by the negative potential of the voltage source. The charge carriers are carrying heat to the junction of the P-type and N-type material, thus the device is providing a heating function at the junction of the N-type and P-type materials.
Conversely, when the thermoelectric device is connected to a voltage source such that the N-type material is connected to the positive lead of the voltage source and the P-type material is connected to the negative lead of the voltage source, the opposite effect occurs. Charge carriers (electrons) in the N-type material are repelled by the negative potential and attracted to the positive potential of the voltage source. Similarly, the positive charge carriers (holes) in the P-type material are repelled by the positive voltage potential and attracted by the negative potential of the voltage source. The charge carriers are carrying heat away from the junction of the p-type and n-type material, thus the device is providing a cooling function at the junction of the P-type and N-type materials.
Given the pairing of N-type material with P-type material, it is common to believe that thermoelectric devices will function in a manner similar to a diode. However, this is not the case, since in a diode a depletion region is formed between the P-type material and the N-type material. When the diode is forward biased, charge carriers are drawn into the depletion region and the diode becomes conductive. When the diode is reverse biased charge carriers are drawn away from the depletion region and the diode acts as an open circuit. The thermoelectric device does not form a depletion region and therefore does not function in a manner similar to a diode. The thermoelectric device conducts in both directions and there is only a small voltage drop across the device.
Although certain thermoelectric materials have been known in the art for a number of years (e.g.—bulk semiconductors), it has only recently been found that thermoelectric materials having a superlattice structure can possess thermoelectric properties which are better than the corresponding thermoelectric properties of other thermoelectric materials.
A superlattice structure denotes a composite structure made of alternating ultrathin layers of different component materials. A superlattice structure typically has an energy band structure which is different than, but related to, the energy band structures of its component materials. The selection of the component materials of a superlattice structure, and the addition of relative amounts of those component materials, will primarily determine the resulting properties of a superlattice structure as well as whether, and by how much, those properties will differ from those of the superlattice structure's component material antecedents.
It is generally known that thermoelectric materials and thermoelectric materials having a superlattice structure find application in the fields of power generation systems, and the heating and/or cooling of materials. One problem, however, is that although these fields place ever-increasing demands on thermoelectric materials to possess ever-improving thermoelectric performance characteristics, the thermoelectric materials and thermoelectric materials having a superlattice structure known in the art have, as of yet, not been able to keep pace with such performance demands.
One way to predict the thermoelectric behavior of thermoelectric materials or thermoelectric materials having a superlattice structure in the fields of power generations systems, and the heating and/or cooling of materials is to calculate a thermoelectric figure of merit for the materials. The thermoelectric figure of merit, ZT, is a dimensionless material parameter in which T corresponds to temperature and Z is the figure of merit. ZT is a measure of the utility of a given thermoelectric material or thermoelectric materials having a superlattice structure in power generation systems, and heating and/or cooling applications at a temperature T.
The relationship of ZT to the material properties of thermoelectric materials and thermoelectric materials having a superlattice structure is shown by the following equation:
The dimensionless materials (or intrinsic) TE figure of merit ZT is defined as followsZT=S2T/ρκ  (1)
where T is the temperature, S is the Seebeck coefficient or thermopower, ρ is the electrical resistivity, and κ is the thermal conductivity.
Generally, it is known in the art that it is desirable for thermoelectric materials to have a relatively high value for their thermoelectric figure of merit (ZT) in order for those thermoelectric materials to perform well in the fields of power generation systems and the heating and/or cooling of materials. From inspection of the above equation, it appears that to provide a thermoelectric material having a high ZT, one need only fabricate on it a superlattice structure having relatively high values for its Seebeck coefficient and its temperature while, at the same time, having a relatively low value for its thermal conductivity and resistivity. It has proven difficult in practice to provide a thermoelectric material or a thermoelectric material having a superlattice structure that has a high thermoelectric figure of merit (ZT) value.
As is also known in the art, multilayer systems prepared by molecular beam epitaxy (MBE) can provide materials having improved thermoelectric properties. Superlattice systems having reduced dimensionality have been proposed as a means to greatly enhance the thermoelectric figure of merit (ZT) as a result of the effects of confinement on the electronic density of states. It has also been shown that additional effects need to be included in order to obtain a more complete understanding of these complex structures.
The above discoveries have led to increasing interest in quantum-well and quantum-wire superlattice structures in the search to find improved thermoelectric materials for applications in cooling and power generation. Investigation of Pb1−xEuxTe/PbTe quantum-well superlattices grown by MBE yielded an enhanced ZT due to the quantum confinement of electrons in the well part of the superlattice structure have been conducted.
Quantum wells have two-dimensional carrier confinement whereas quantum wires have one-dimensional confinement of the carriers. Quantum wires have been calculated to have much higher ZTs than quantum wells due to improved confinement. And, it has been recognized that quantum dots (QDs) have even higher ZT values than quantum wires.
Quantum dots have zero-dimensional confinement and represent the ultimate in reduced dimensionality, i.e. zero dimensionality. The energy of an electron confined in a small volume by a potential barrier as in a QD is strongly quantized, i.e., the energy spectrum is discrete. For QDs the conduction band offset and/or strain between the QD and the surrounding material act as the confining potential. The quantization of energy, or alternatively, the reduction of the dimensionality is directly reflected in the dependence of the density of states on energy. For a zero-dimensional system (e.g. a QD superlattice), the density of states (dN/dE) of the confined electrons has the shape of a delta-like function
      (                  ⅆ        N            /              ⅆ        E              )    ⁢  α  ⁢            ∑              ɛ        i              ⁢          δ      ⁡              (                  E          -                      ɛ            i                          )            
where εi is discrete energy level and δ is the Dirac function. Thus, an enhanced density of states is a possibility even in partially confined QD superlattice (QDSL) structures.
Solid state thermoelectric (TE) cooling and electrical power generation devices can be fabricated using QDSL materials. TE devices have many attractive features compared to other methods of refrigeration/electrical power generation. These features include long life of the device, an absence of moving parts, non-emissions of toxic gases (hence environmentally friendly), low maintenance, and high reliability. Despite the features associated with TE devices however, widespread use in many applications has been limited by relatively low energy conversion efficiencies.
Quasi-zero-dimensional Quantum dot Superlattice (QDSL) structures, having a delta-function distribution of density of states and discrete energy levels due to three-dimensional quantum confinement, a potentially more favorable carrier scattering mechanism, and a much lower lattice thermal conductivity, provide the possibility of much more efficient thermoelectric devices. The growth of self-assembled QDSL materials on planar substrates using the Stranski-Krastanov growth mode, yields improved TE figures of merit. Epitaxially grown PbSeTe/PbTe (ternary) QDSL materials yielded a conservatively estimated room temperature TE figures of merit of 0.9.